I'm sitting here trying to think of a single other more offensive concept than the idea of a "sin nature" in Christianity, and I'm having a little trouble thinking of any. Rampant misogyny? Advocacy of slavery? Genocidal marching orders? Oh, God didn't legislate against those things because humans are fallen and had to have a little leeway. Can't stop masturbating? Lusting after gay sex? Cheating on your taxes? Cussing? Pounding on the horn in daily road rage? Of course you'll see Christians having trouble in those arenas. We have a sin nature, you see, so we can't totally change overnight even with the power of our almighty Lord.
Seriously, they don't even recognize how completely crazy that sounds. I sure didn't, till I left the faith. Here is why it's crazy:
Christians believe that once they convert and are baptized or whatever their particular denomination says they have to do to be really and truly saved, they're totally different. Reborn. Renewed. Washed clean. The idea is that once converted, a Christian becomes this powerhouse of morality and purity. They are filled and possessed by the holiness of an omnipotent, utterly good god, and so cannot be impure. The problem is that this idea is total nonsense. The reality of how Christians behave is a lot closer to how an abuser acts when finally caught out: there's a honeymoon period of presents and strictly-self-monitored behavior, and then a little while later the old patterns re-emerge.
Even the Bible wrestles with this complete breakdown in logic. "Easier for a leopard to change its spots" battles with "even as dogs go back to their own vomit." Paul reveals in Corinthians that he battled a "thorn in his flesh," but preaches in Romans that Christians are "controlled not by the sinful nature but by the Spirit, if the Spirit of God lives in you." Even then they saw that it was all bullshit but couldn't admit it.
Modern Christians take the concept and run with it as a "get out of jail free" card for their behavior, a card given to them by their Buddy Jesus. Even one of ex-Christian.net's resident fundies once rationalized his unacceptable behavior on a forum by mewling about his "sin nature" getting temporarily out of control, not even realizing how weak that made his god look to the rest of us. They may cry afterward about how sad they were about doing the offense, and they may chirp about how being Christian means they're not perfect but forgiven, but don't ever seem to grasp that the offense should have been physically impossible for someone truly possessed by an all-knowing, all-benevolent God.
If God really were real, then we would expect his adherents to be among the most moral, upright, and staunchly good people in the world. What we see is the polar opposite. And I blame this dichotomy upon the "sin nature" concept that modern Christians cling to to excuse their utter inability to act like the Bible says they should. There is no transformation. There is no true change. If someone was a fundamentally good person before her conversion, then she'll still be a decent person afterward; if someone was a lying sack of shit beforehand, he'll still be a liar afterward. The liar may struggle to "be a good boy" after a big conversion experience (or a "renewing" one at a revival or refilling or whatever word they're using nowadays to describe an extra-special preaching week), but he'll falter and revert to his old ways before too long.
But there cannot be any "old ways" remaining if Buddy Jesus has possessed the liar. There can't be any of that sin left. The Christian cannot have it both ways, no matter what he thinks. He can't have a "sin nature" and yet be possessed by Jesus. He can't commit the most horrific of sins and expect that he'll be acceptable to Jesus in the Forever Playground, yet still firmly believe that the smallest sin committed by a non-believer would damn her forever to hell.
There's a level of self-delusion and egotism inherent in the idea of a "sin nature" that is staggering to behold. Everybody BUT the Christian spouting that tired catchphrase sees how ridiculous it is, and how illustrative of the utter powerlessness and impotence of their god. But not only do Christians spout this idea, they cling to it--and cling they must, because without it, their god loses a lot of his appeal. Already we know his "miracles" are easily explained by urban legend proliferation, picayune happenstance, or confirmation bias--so if he also cannot actually make his followers better people even with the threat of hell to back him up, there isn't much else to bring in the outsiders and their tithe money.
I rather suspect (in my heart of hearts) that Christians are such spectacular failures by their own standards because they know deep down that there is no hell, no heaven, and no god judging them after death. Just as a child with an ineffective parent acts out even worse because she knows nothing will really happen to her if she does so, a Christian with an impotent, ineffective God will do worse and worse things because he knows that it isn't really true. But there they are crowing about how Christians are better people and are so much more pure and sweet and kind than non-Christians!
Occam's Razor strikes yet again:
There is no such thing as "sin nature" because there is no Jesus or Yahweh to sin against. A believer can try very hard to change, but if any change does occur, it does so by the power of the human will and simple hard work. Though the Bible says that belief in Jesus will change a person for the better, Christians don't act, as a group, better than anybody else and in fact act a lot worse; the rationalization about "sin nature" has created a style of Christianity that can both excuse away insiders' shortcomings and damn outsiders even more than before.
For a God, this one has some awfully tinny feet.